FABP4 (aP2) and FABP5 (mal1) are members of the fatty acid binding protein family. FABPs are proteins of 14-15 KDa that act as chaperones for fatty acids in the aqueous cytosolic environment and facilitate their movement between cellular compartments. So far at least nine members of this family have been identified with a tissue-specific pattern of expression. FABP4 is mainly expressed in adipose tissue and macrophages, but also in other cell types, whereas FABP5 is expressed in a wide range of tissues and organs. FABPs are responsible for the transfer of fatty acids to different cell compartments and are thus implicated in key cellular functions such as lipid storage in adipocytes, fatty acid oxidation in mitochondria, ER signaling, fatty-acid-dependent gene expression, regulation of cytosolic enzymes activity, modulation of inflammatory response and leukotriene synthesis. Plasma FABP4 is secreted by adipose tissue in mice and secretion is de-regulated in obesity and blocking of plasma FABP4 in vivo by antibodies improves insulin sensitivity.
Several genetic evidences in human support a role of FABP4 and FABP5 in metabolic diseases. A mutation in the FABP4 promoter (SNP T-87C) leading to 50% reduction in gene expression is associated to reduced cardiovascular diseases (CVDs) and type 2 diabetes (T2D) risk and to reduced plasma triglycerides (TGs). Two mutations in FABP5 gene, one in the 5′UTR (rs454550), one in the promoter (nSNP), are associated, respectively to increased (OR 4.24) and decreased risk (OR 0.48) of T2D. In addition, it was shown that FABP4 protein and mRNA levels in atherosclerotic plaque macrophages are associated to plaques instability and CV death. Finally, a large number of publications report an association between FABP4 and FABP5 plasma levels and severity of metabolic diseases. Elevated FABP4 plasma levels are associated with atherogenic dyslipidemia, reduced endothelial function, increased intima-media (IM) thickness, metabolic syndrome, obesity and insulin resistance IR. Elevated FABP5 plasma levels are associated to metabolic syndrome.
Genetic and pharmacological studies in mice largely confirm the human evidences. It was demonstrated that loss-of-function in FABP4 and FABP5 improves insulin sensitivity, lowers glucose, and protects against atherosclerosis. FABP4 knockout mice on high fat diet showed metabolic improvement that was tempered by compensatory upregulation of FABP5 in adipose. Mice with a deletion of FABP5 gene on high fat (HF) diet showed body weight reduction and improved glucose and insulin tolerance. The FABP4/FABP5 double-knockout mice were strongly protected from hyperglycemia, insulin resistance, and hepatic steatosis. In addition, in an ApoE deficient background, FABP4 and FABP5 deletion was highly protective against the development of atherosclerosis and increased longevity. A specific FABP4 inhibitor (BMS309403), showed in a clamp study in ob/ob mice a reduction of hepatic glucose production, increased glucose uptake in muscle and adipose and reduction in hepatic steatosis, but no change in body weight and energy consumption. Also, it showed a decrease in atherosclerotic placques formation in ApoE KO mice. A dual FABP4/5 inhibitor, Compound 3 described in J. Lipid Res. 2011, 52, 646, showed in mice under HF diet a reduction in plasma triglycerides and free fatty acids, but no improvement in insulin and glucose tolerance.